Underground drilling involves drilling a bore through a formation deep in the Earth using a drill bit connected to a drill string. During drilling, an autodriller control system may be used to control the rate of penetration of the drill bit at the bottom hole assembly on the drill string. The rate of penetration may be based on a control parameter as a set point, such as weight on bit or surface differential pressure of the drilling fluid. For example, when measured surface differential pressure is used as a set point, the autodriller control system may reduce the weight on bit as measured surface differential pressure increases. Conversely, the autodriller control system may increase the weight on bit as the measured surface differential pressure decreases.
As the drill bit cuts into the surrounding formations in the wellbore, cuttings are produced. These cuttings mix with the drilling fluid (also referred to as drilling mud) in an annulus between the drill string and the sides of the wellbore. The drilling fluid transports these cuttings during circulation, eventually evacuating with the drilling fluid from the wellbore. However, as cuttings are added to the drilling fluid, this adds density to the drilling fluid, resulting in added pressure at the bottom of the wellbore at the bottom hole assembly. This is detected as an increase generally in the surface differential pressure that is attributed by existing control systems to increased pressure from the mud motor—but does not reflect an actual increase in mud motor torque.
As a result, present approaches respond by backing off one or more drilling parameters, such as weight on bit or block running speed, and therefore slowing the rate of penetration, in situations where it is not warranted by actual downhole conditions. The present disclosure is directed to systems, devices, and methods that overcome one or more of the shortcomings of the prior art.